A mobile handover process takes place when a mobile communications device (user terminal) is required to cease communicating with a core network through one access node (also known as a base station) and begin communication through another access node. The most common reasons for such handovers to be required are because either the user terminal or the access node detects deterioration of the signal quality on the wireless communications link between them. This can be because the mobile device is moving out of range of the access node, but other changes in the wireless environment, such as changes in congestion or interference levels, may also make a handover appropriate. Handover may also occur when a user “roaming” on a network other than his “home network” (the one to which he subscribes) moves into range of an access node of his home network: in such a case a handover to the home network is desirable as soon as signal quality between the user terminal and the home network meets a predetermined threshold, regardless of the signal quality on the other network, because this will allow the user to use any facilities specific to his “home” network, and avoid paying the higher charges usually required for connection through a network other than the user's home network.
Unless the context requires otherwise, the term “access node” should be interpreted in this specification to mean any device or apparatus with which a terminal may communicate wirelessly in order to allow the terminal to communicate with a backhaul connection to a core communications network. It includes, for example, access points (wireless routers) for “WiFi” (IEEE 802.11 standard) access networks, as well as the base stations used in cellular telephony.
The decision to initiate a handover, and the selection of a new link to which to hand over, is typically based on signal strength and a connection is established with the access node generating the strongest signal (subject to the access node's capacity and authority to accept a connection with the mobile unit).
For present purposes the access node to which the user terminal is initially connected will be referred to as the “source” node, and the node to which handover is to be made will be referred to as the “target” node. There is in general no significant difference in practice between the nodes—in any particular session an individual node may be the target node in a first handover and the source node in a subsequent handover, either back to the original source node (now acting as a target node) or to a third node.
Embodiments will be described herein using the terminology of the 4G or Long Term Evolution (LTE) standard, but the underlying principles are applicable to other mobile communications systems and the use of this terminology should not be taken as limiting.
In most network architectures the connections between individual access nodes (also known as base stations or, in the LTE standard, as eNodeB's or eNBs) and the rest of the network are controlled by access control systems, also known as in the LTE standard as MMEs (mobile management entities), Typically, several access nodes may be controlled by the same access control system, and when a handover is arranged between two of those nodes the whole process can be managed by the access controller. In a cellular system with access nodes in permanent locations, it is conventional to maintain a “neighbor list” for each access node which can be used to inform the mobile unit of the base stations to which handover is most likely to be possible. However, when both source and target access nodes are associated with the same access control system, and a suitable interface exists between the nodes, a so-called X2-based handover can be affected. This enables faster handover for latency-critical applications such as voice services and gaming as well as less load on the core network components.
However, in the general case, if the source and target nodes are associated with different access control systems (MMEs) there is normally no direct signaling interface between the source and target access nodes and all handover messages are conventionally passed via the access control systems (MMEs) associated with the respective access nodes (herein referred to respectively as the source access control system or source MME and the target access control system or target MME), and the data path is also tunneled by way of the respective source and target Service Gateways (SGWs) through which the respective access nodes are connected to the rest of the network. This implies longer latency for data traffic during handover as well as longer handover times and additional load on the core network. As national roaming agreements are becoming more common (that is, handing off between different operators in the same region) handovers between different access control systems (MMEs) are becoming more common. For example when a user leaves a home environment where he has a private hotspot connection from his service provider into a public wide-area macrocell operated by a cellular partner, the signal strength drops very rapidly so fast handover is needed to maintain the session.
If the MMEs are both controlled by the same user plane entity, as is described for example in United States Patent App. Pub. No. US2007/254667 (Jokinen), the handover can be mediated by that user plane, which can determine whether both the target and source MMEs support the “X2”-based handover. However, in general the MMEs will be associated with different control planes and will therefore not have visibility of each other's capabilities. In the absence of such information, the MMEs default to the less efficient “S1” handover process in which data is redirected by way of a tunnel from one MME to the other by way of the gateways.